Underground tanks are frequently used for the storage of liquids, such as gasoline at service stations. Such tanks can be made of fiberglass, coated steel or other materials. While the integrity of most tanks is generally good, leaks occasionally occur resulting in the product contained within a tank escaping into the environment and/or ground water seeping into the tank and contaminating the product. Environmental concerns and resulting governmental regulations have led to the widespread use of double-wall tanks having an inner wall to contain the product and an outer wall surrounding the inner wall, with a small space separating the two.
The double-wall configuration serves two purposes. First, if a leak forms in the inner wall, the outer wall will prevent product from escaping into the external environment and possibly polluting the soil or ground water. Secondly, if a leak forms in the outer wall, the inner wall will prevent ground water from entering the inner tank and contaminating the product. Consequently, a breach in either wall will result in product or ground water being retained in the annular space between the walls, which is typically about 1 inch to 11/4 inches wide (although tanks with larger and smaller spacing are also used).
It can be appreciated that the detection of product or water within the annular space is extremely important. If a leak is detected and confirmed quickly, the tank can be repaired or, more typically, replaced before significant contamination occurs. Various devices have been proposed for detecting leaks, such as filling the annular space with a fluid and monitoring the hydrostatic pressure or electrical conductivity of the fluid. Another method employs a dye which is soluble in hydrocarbons but not water. The dye is lowered to the bottom of the annular space and periodically removed for inspection. If a leak of hydrocarbon liquid has occurred, the dye will have dissolved.
U.S. Pat. No. 4,682,156 by Wainwright, issued Jul. 21, 1987 and entitled "Solvent Detector," commonly assigned to the assignee of the present invention, is directed to the detection of solvents, including hydrocarbons, in a perforated well casing. The device employs a hydrocarbon-soluble filament which interconnects a mass and a weighing assembly, the latter being joined to an alarm indicator. The filament and mass are lowered into the well casing; hydrocarbons present in the space will dissolve the filament and allow the mass to fall free. The weighing assembly detects the resulting reduction of weight and triggers an alarm.
U.S. Pat. No. 4,712,505 by Wainwright, issued Dec. 15, 1987 and entitled "Combination Hazardous Liquid and Water Sensor," also commonly assigned to the assignee of the present invention, discloses an apparatus and a method for sensing the presence of product and/or water in the annular space between the walls of a double-wall tank. A product-sensing element and a water-sensing element are operatively associated with each other to control the compression of a spring. The spring is operatively connected to a colored marker which is able to move from within an opaque tube to within a transparent tube using the force of the spring. When the product-sensing element is contacted by product or the water-sensing element is contacted by water, the respective element weakens and the spring force moves the colored marker so that it is within the transparent tube to indicate that product or water is present in the annular space.
It can be appreciated that some sensors will not detect the presence of water or are not sensitive to the presence of small amounts of product or water. Conversely, with some sensors, particularly those designed to be particularly sensitive to the presence of even very small amounts of product or water, condensation which forms in the annular space can trigger false indications of leakage. Such condensation can form, for example, when the temperature of the product within the inner tank is different from the temperature of the ground outside the outer tank. This temperature difference may occur when product is added to the tank or when a manhole cover above a tank riser is warmed, such as by the sun. Condensation tends to form first on rough surfaces, such as the surfaces of the walls of a double-wall tank or on the surface of the cable used to connect a remote indicator to a sensor located at the bottom of the annular space. Such a cable typically is routed down the tank riser to the bottom of the annular space. Because the distance between the inner and outer walls may only be about 1 inch, the cable will generally follow the circumference of the outer wall. Consequently, condensation which forms within the annular space can run down either or both of the tank walls to the bottom of the annular space and can also run down the cable to the sensor. Furthermore, tanks are frequently installed in the ground with a slight angle to horizontal (about 1.degree. to 2.degree.) in order to enhance the pooling of leaked product or water to facilitate detection. However, pooling of condensation is also enhanced, increasing the likelihood that condensation will be detected by the sensor and producing a false indication of leakage.
Because an operator cannot determine in advance whether an indication of water in the annular space is a result of a leak or a result of condensation, all such indications must be checked out. This typically involves pulling the sensor assembly out from the annular space and visually inspecting it. And, because typical sensor elements can only be used once, replacement of the sensor element is required after contact with condensation. The sensor assembly must then be reinstalled in the bottom of the annular space. Manual inspection is time consuming and inconvenient and replacement of sensor elements is an undesired expense. Consequently, a need has arisen for a device and a method for reducing false indications of leakage due to condensation between the inner and outer walls of a double-wall tank.